| Direction: The bond dissociation energy of a diatomic molecule is also called bond energy. Bond energy is also called, the heat of formation of the bond from the gaseous atoms constituting the bond with reverse sign. |
| Example\[H(g)+Cl(g)\xrightarrow{\,}\,H-Cl(g),\]\[\Delta {{H}_{f}}=-431\,\,kJ\,\,mo{{l}^{-1}}\]or bond energy of \[H-Cl=-(\Delta {{H}_{f}})\]\[=-(431)=+431\,kJ\,mo{{l}^{-1}}\] When a compound shows resonance there occurs a fair agreement between the calculated values of heat of formation obtained from bond enthalpies and any other method. However deviation occur incase of compounds having alternate double bonds. |
| Example \[\underset{(g)}{\mathop{{{C}_{6}}{{H}_{6}}}}\,\xrightarrow{\,}\underset{(g)}{\mathop{\,6C}}\,+\underset{(g)}{\mathop{6H}}\,\] |
| Resonance energy = experimental heat of formation - calculated heat of formation |
| If (i) \[\Delta H_{f}^{o}\] (benzene) \[=-358.5\,\,kJ\,\,mo{{l}^{-1}}\]. |
| (ii) Heat of atomization of graphite \[=716.8\,\,kJ\,mo{{l}^{-1}}\]. |
| (iii) Bond energy of \[C-H,\,\,C-C,\,\,C=C\] and \[H-H\]bonds are 490, 340, 620 and\[436.9\,\,kJ\,\,mo{{l}^{-1}}\] respectively. The resonance energy (in \[kJ\,\,mo{{l}^{-1}}\]) of \[{{C}_{6}}{{H}_{6}}\] using Kekule formula is |
A) - 150
B) - 50
C) - 250
D) +150
Correct Answer: A
Solution :
\[6C(g)+3{{H}_{2}}(g)\xrightarrow{\,}\,{{C}_{6}}{{H}_{6}};\] \[\Delta {{H}_{\exp }}=-358\,kJ\] \[\Delta {{H}_{f}}\] can also be calculated as; \[\Delta {{H}_{f}}({{C}_{6}}{{H}_{6}})=[6\times \Delta {{H}_{C}}(s)\xrightarrow{{}}\] \[{{C}_{(g)}}+3\times \Delta {{H}_{H-H}}]\] \[-[3BE\,of\,C=C+3BE\,of\,C=C\,+6BE\,of\,C-H]\] \[=[6\times 716.8+3\times 436.9]\] \[-[3\times 340+3\times 620+6\times 490]\] \[\Delta {{H}_{resonance}}=\Delta {{H}_{f}}_{(\exp )}-\Delta {{H}_{f(calc)}}\] \[=-358.5-(-208.5)=-150\,kJ\,mo{{l}^{-1}}\]
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